1. Field of the Invention
The present invention relates to a voltage controlled oscillator for controlling an output signal frequency in accordance with an input voltage, and particularly, to a voltage controlled oscillator having improved C/N characteristics.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. 8-148933 discloses a conventional voltage controlled oscillator, the objective of which is to improve C/N characteristics. This device will be explained with reference to FIG. 3.
In general, a voltage controlled oscillator may be used for mobile communication equipment, for use in the 900 MHz band for example. The oscillator basically comprises: a resonance circuit 1 having a resonance frequency which changes according to a control voltage applied to a control terminal C; an oscillation circuit 2 having an oscillation frequency which is determined depending on the resonance frequency of the resonance circuit 1; a buffer circuit 3 for amplifying a signal outputted from the oscillation circuit 2 and for preventing an oscillation frequency from fluctuating due to load fluctuation; and an output matching circuit 4 for matching the impedance of an external circuit, not shown, connected to the output terminal P and for suppressing a higher harmonic component. Each circuit component of the voltage controlled circuit thus configured is formed by incorporating a plurality of electronic component functions to be described later on or inside a multilayered circuit board formed by simultaneously baking a plurality of laminated ceramic sheets, each having a circuit pattern printed thereon.
The resonance circuit 1 comprises a coupling capacitor C1, a variable-capacitance diode VD, a resonance inductor L1, and a resonance capacitor C2. A control voltage VC applied to the control terminal C is supplied to the variable-capacitance diode VD via a choke coil L2. Note that the control terminal C is grounded with respect to a high frequency by a high frequency bypass capacitor C3.
The oscillation circuit 2 comprises an oscillation transistor Q1 as an active component, bias resistors R1, R2, and R3, Colpitts capacitors C4 and C5, a high frequency bypass capacitor C6, a strip line SL connected in series with the bias resistor R3, and a chip capacitor CC connected in parallel with the strip line SL. The strip line SL and the chip capacitor CC constitute a parallel oscillation circuit. The oscillation circuit 2 is connected to the resonance circuit 1 via a coupling capacitor C7.
Additionally, the bias resistor R3 determines the direct current bias of the emitter as a current output terminal of the oscillation transistor Q1.
The buffer circuit 3 comprises a buffer transistor Q2 and bias resistors R4 and R5. The buffer circuit 3 is connected to the oscillation circuit 2 via a coupling capacitor C8.
The output matching circuit 4 comprises a choke coil L3, a coupling capacitor C10, a high frequency bypass capacitor C11, and the output terminal P.
Note that a driving power supply terminal B provides a voltage VB as a driving power source to the oscillation transistor Q1, and the buffering transistor Q2 is grounded with respect to a high frequency by a bypass capacitor 12.
Additionally, a resonance frequency of the resonance circuit 1 changes by changing the capacitance of the variable capacitance diode VD in response to a value of a control voltage VC. Further, an oscillation circuit 2 oscillates at the resonance frequency, and its signal is outputted from the terminal P.
The Q of the resonance circuit 1 is most significantly affected by the impedance of the emitter of the oscillation transistor Q1, which is a load of the resonance circuit 1. The higher the impedance of the emitter, the lower the amount of degradation of the Q of the resonance circuit 1. This results in improvement in the C/N characteristics of the voltage controlled oscillator.
In a voltage controlled oscillator thus configured, in a parallel resonance circuit comprising the strip line SL and the chip capacitor CC, the resonance frequency of the strip line SL and the chip capacitor CC is made to match a predetermined frequency to raise the impedance of the emitter of the transistor Q1 at a predetermined frequency by separately changing the inductance of the strip line SL and the capacitance of the chip capacitor CC. This improves the C/N characteristics of the voltage control oscillator.
However, in the conventional voltage controlled oscillator described above, generally, the resonance inductance L1 of the resonance circuit 1 is configured by a strip line resonator. Accordingly, there are two strip lines, i.e., a strip line resonator as the resonance inductance L1, and a strip line SL within the oscillation circuit 2.
These strip lines are formed as a circuit pattern on a multilayered circuit board (not shown). In order for both of the strip lines to secure a sufficient characteristics, a certain degree of length is required. Thus, to maintain sufficient characteristics, enlargement of the multilayer circuit board is required. Since the size of the voltage control oscillator itself is enlarged, it has been difficult to meet the market demand for compactness of the voltage controlled oscillator.
Additionally, since a strip line resonator and a strip line SL are provided in one multilayered circuit board, electric bonding (e.g., coupling) between strip lines is generated. As a result, Q in this resonance system is lowered, and a C/N characteristics are degraded.
Hence, the present invention is made to solve at least the problem described above, and one object of the present invention is to provide a voltage controlled oscillator for improving C/N characteristics and which can be easily made more compact.